Great question, and the answer to which I didn't have so had to buy a lock removal tool.
Worst part, I had put the new locking lugs on and immediately lost the lock and the ow almost 2 years later I still have no idea where the f it went. Seriously, in the driveway and realized I needed it an could not find it. Damndest thing.
I'm thinking that several of the responses are about spline adapters and not locking keys since a set of locking nuts only comes with one key. I stopped using my locks after I got the Pro Comp wheels. The bore on the wheels are small and I have to use spline lug nuts and even though I could just fit the lock nut in it was a pain. Last thing I want is not be able to get the wheel off on the trail anyway because I can't find the key. I keep the spline adapter in the little plastic tray over the scissor jack. I should get about ten more of those adapters and put them everywhere.
As others have mentioned, mine lives in the rear cubby as well. It is reasonably secure as I have a Tuffy 0326 storage box that covers the lid for the cubby and I do keep the rear gate locked.
Having come of age in the 35mm film days, I still have a dew of the plastic film containers (Sadly, almost all of my metal ones are so worn as to be non-serviceable) and the lock key is kept in one to keep it clean, prevent damage, and to make it a bit easier to find as I seem to have about half of my worldly possessions crammed in that cubby.
I'm in the "get rid of them" camp.. I do my own regular tire rotations. It's a pain messing with the locking lug nuts.
It might be an advantage to use a single locking nut on the spare wheel/tire only. The spare is very easy to steal since you don't have to jack up the vehicle. A crook wanting to replace a blown, torn up tire could whip off the spare and be gone in under 30 seconds.
Put mine in the cubby back with the jack. If it makes you feel any better, I didn't get one with my Jeep. Trip to the auto parts store and six bucks later I had one. So (1) they prolly aren't much of a deterrent and (2) if you can't find it then a trip to the auto parts store and you're good to go.
I have mine in the glove box. When I take my Jeep in for the free oil changes and tire rotation, I put it in the pocket above the radio. Now, I always check to make sure they have put it back. I've driven all the way home before looking for it and they did not put it back.
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